Auto focus system and auto focus method for use in video cameras

Abstract

An exemplary auto focus method for use in video cameras including a focus lens includes: focusing the pickup lens and capturing a current image and a prior image; calculating two contrast estimation values of the current image and the prior image; and judging whether a difference between the contrast estimation value of the current image and that of the prior image exceeds a predetermined threshold, if yes, refocusing the pickup lens, if no, keeping focus status of the pickup lens unchanged and capturing a subsequent image.

1 . An auto focus system for use in a video camera comprising:
 an image sensor for capturing a plurality of consecutive images including a current image and a prior image of an object;   a pickup lens including a focus lens for focusing light from the object on the image sensor;   a calculating unit for calculating a contrast estimation value of each of the images;   a driving unit for moving the focus lens to focus the pickup lens; and   a focusing controller for judging whether a difference between the contrast estimation value of the current image and that of the prior image exceeds a predetermined threshold, if yes, activating the driving unit to focus the pickup lens, if no, deactivating the driving unit.   
   
   
       2 . The auto focus system as claimed in  claim 1 , wherein the pickup lens comprises a lead screw and a step motor; the focus lens being engaged with the lead screw; the step motor being configured for driving the lead screw to rotate to force the focus lens to move stepwise along the optical axis of the focus lens. 
   
   
       3 . The auto focus system as claimed in  claim 1 , wherein the focus lens is restricted to move within a focus range and in which the focus lens is adjustably positioned at one of a plurality of focus positions. 
   
   
       4 . The auto focus system as claimed in  claim 1 , wherein the pickup lens comprises a zoom lens, the zoom lens and the focus lens being coaxially arranged, the zoom lens being slidable along the optical axis thereof to change the focal length of the pickup lens. 
   
   
       5 . The auto focus system as claimed in  claim 1 , wherein the image sensor is selected from a group of charge coupled device image sensor and complementary metal oxide semiconductor image sensor. 
   
   
       6 . The auto focus system as claimed in  claim 1 , further comprising an extracting unit for extracting a brightness component from each of the images. 
   
   
       7 . The auto focus system as claimed in  claim 1 , wherein the calculating unit is configured for calculating a contrast value array of each of the images according an equation:
     e   mn   =|g   mn   −g   m(n−1)   |+|g   mn   −g   m(n+1)   |+|g   mn   −g   (m−1)n   |+|g   mn   −g   (m−1)n |,   
     and the contrast estimation value is a function of the contrast value array, where e mn  is an element of the contrast value array at the intersection of line m and row n, g mn  is the grayscale value of a pixel of the image at the intersection of line m and row n, and m, n are positive integer. 
   
   
       8 . The auto focus system as claimed in  claim 1 , further comprising:
 a dividing unit for dividing the image into a plurality of areas, the calculating unit calculates a plurality of contrast estimation values of the respective areas to form an area estimation value array;   a weighting unit for weighting the area estimation value array using a weighting factor array and an equation:
     we   mn   =es   mn   ×w   mn    
   
     to obtain a weight area estimation value array, where we mn  is an element of the weighted area estimation value array at the intersection of line m and row n, es mn  is an element of the area estimation value array at the intersection of line m and row n, w mn  is an element of the area estimation value array at the intersection of line m and row n, and m and n are positive integers; the contrast estimation value being a function of the weight area estimation value array. 
   
   
       9 . The auto focus system as claimed in  claim 8 , further comprising a memory for storing the calculated contrast estimation values, the driving unit and the focusing controller read the calculated contrast estimation values from the memory. 
   
   
       10 . An auto focus method for use in a video camera having a focus lens comprising:
 focusing the pickup lens and capturing a current image and a prior image;   calculating two contrast estimation values of the current image and the prior image; and   judging whether a difference between the contrast estimation value of the current image and that of the prior image exceeds a predetermined threshold, if yes, refocusing the pickup lens, if no, keeping focus status of the pickup lens unchanged and capturing a subsequent image.   
   
   
       11 . The auto focus method as claimed in the  claim 10 , further comprising:
 extracting a brightness component from each of the images, the contrast estimation value is calculated based upon the brightness component.   
   
   
       12 . The auto focus method as claimed in  claim 10 , wherein the pickup lens comprises a focus lens, focusing the pickup lens comprising:
 driving the focus lens of the pickup lens to move through a plurality of focus positions to obtain a plurality of images corresponding to the plurality of focus positions;   calculating a plurality of contrast estimation values of the plurality of images;   comparing the plurality of contrast estimation values to find the highest contrast estimation value; and   driving the focus lens to move to the focus position where the respective image with the highest contrast estimation value was obtained.   
   
   
       13 . The auto focus system as claimed in  claim 10 , wherein the contrast estimation value is calculated by calculating a contrast value array based upon a brightness component of the image using an equation
     e   mn   =|g   mn   −g   m(n−1)   |+|g   mn   −g   m(n+1)   |+|g   mn   −g   (m−1)n   |+|g   mn   −g   (m−1)n |,   
     and the contrast estimation value is a function of the contrast value array, where e mn  is an element of the contrast value array E at the intersection of line m and row n, g mn  is the grayscale value of a pixel of the image at the intersection of line m and row n, and m and n are positive integers. 
   
   
       14 . The auto focus system as claimed in  claim 13 , wherein the contrast estimation value is the sum of all elements of the contrast value array. 
   
   
       15 . The auto focus system as claimed in  claim 10 , wherein calculating a contrast estimation value comprising:
 dividing the image into a plurality of areas, and calculating a plurality of contrast estimation values of the plurality of areas to form an area estimation value array;   weighting the area estimation value array using a weighting factor array and an equation
     we   mn   =es   mn   ×w   mn    
   
     to obtain a weight area estimation value array, where we mn  is an element of the weighted area estimation value array at the intersection of line m and row n, es mn  is an element of the area estimation value array at the intersection of line m and row n, w mn  is an element of the area estimation value array at the intersection of line m and row n, and m and n are positive integers; the contrast estimation value being a function of the weight area estimation value array. 
   
   
       16 . The auto focus system as claimed in  claim 15 , wherein the contrast estimation value is the sum of all elements of the weight area estimation value array.